I fell down a Wikipedia Hole deep into the history of nuclear reactors in space. There are many radioisotope heater units (RHUs) in space, such as those used by Chang'e 4 to survive the cold lunar nights, and radioisotope thermoelectric generators (RTGs), such as the MHW-RTGs used by the Voyagers, the GPHS-RTGs used by Cassini and New Horizons, and the MMRTGs used by Curiosity and Perseverance. (An RTG using 241Am has been proposed for an interstellar mission, as it’s 432 year half-life could provide 500+ years of power [1].) The only nuclear fission reactor launched by the US, and the world’s first, was SNAP 10-A, a 235U - Zirconium hydride reactor with a thermal power output of 30 kW and sodium-potassium (NaK) coolant. It was launched in 1965, stopped working after only 43 days, and has undergone 7 “anomalous events” in which nearly 50 trackable pieces have been seen shedding from the parent body. It should stay in orbit for 3000 years unless it breaks up further. The remaining 30 or so known launched fission reactors were in Soviet RORSATs: nuclear-powered, ocean-surveillance, active radar satellites in LEO that required periodic boosts to prevent reentry. They were designed to launch their enriched 235U reactors into a high orbit at the end of their operational lives, but this didn’t always happen. (Those high orbits will last 300-1000 years, but the nuclear fuel has a half-life of 70,000 years… so future generations may not think highly of us.) In 1978, the Cosmos 954 RORSAT malfunctioned, reentered, and broke apart over northern Canada with the reactor attached, scattering radioactive debris over a 800 km long region and increasing awareness of the challenges around space debris— the USSR eventually paid $3 million in cleanup liability, about a ⅕ of the true cost. Other satellites have also broken up and spread radioactive debris, including the US’s Transit 5BN3 (carried a SNAP RTG) and the USSR’s Cosmos 1402 RORSAT, or have leaked radioactive coolant in orbit, such as the higher power and higher orbit Kosmos 1818 and Kosmos 1867 RORSATs, which both seem to have leaked trails of metallic NaK coolant. The usefulness of the Gamma Ray Spectrometer on NASA’s SolarMax mission was reduced by anomalous gamma ray emissions from these higher reactors. Collisional breakups among the ~30 reactors in orbit are likely over the next few hundred years [2]. Near future use of nuclear power in space may include NASA’s Kilopower reactor and Advanced Stirling Radioisotope Generator, and potentially even nuclear rockets. Related: This 158 page NASA PDF [3] contains a delightfully detailed history of space debris and was a primary source for this writeup.

[1] http://www.niac.usra.edu/files/library/meetings/misc/trieste... [2] https://www.sciencedirect.com/science/article/pii/0273117791... [3] https://web.archive.org/web/20170503083646/https://ston.jsc....

(If you're into this sort of nerdery, I write about this stuff weekly in The Orbital Index - https://orbitalindex.com)